Frame-mounted fascia lata heart valves were used clinically by Doctor Marion I. Ionescu and Doctor D. N. Ross in 1969. Since that time, many such valves made of autologous or hemologous fascia lata, duramater, or heterologous pericardium have been implanted. A construction of bovine pericardial valves is reported in Frame-Mounted Tissue Heart Valves: Technique of Construction, by Ivan T. Bartek, et al., Thorax (1974), 29, pp. 51-55.
This invention relates to an improvement in the structure and fabrication of such a valve as exemplified in the disclosure in U.S. patent application Ser. No. 679,406 now U.S. Pat. No. 4,084,268, by Doctor Ionescu and Bruce Fettel. The prior art exemplified in the above-cited application for letters patent has resulted in great success in the implantation of artificial heart valves. Nevertheless, ways to improve the construction and structure of such valves have been sought after in the art, and specifically ways to improve the structural alignment of the pericardial tissue and the valve stent and improvements which would increase the yield of usable valve tissue from animal hearts. Furthermore, improvements which would decrease the amount of stretch present in the tissue during sewing of the tissue onto the stent were sought after, along with improvements which would eliminate unsymmetrical distribution of stitching bulk on the valve.
In the prior art exemplified in the U.S. patent application Ser. No. 679,406, the valve stent or structural basis has three posts with sewing holes with which the pericardial tissue is sewn to the valve to form three cusps for a tri-cuspid valve. In this prior art, a single integral piece of pericardial tissue is used to form the tri-cuspid valve tissue surface. As is well known in the art, the three cusps must have the same configuration during opening and closing of the valve in order to achieve desirable performance characteristics of the valve. To this end, the three posts are placed 120 degrees apart, and the tissue is sewn onto the post at exactly 120-degree intervals, or at least as close thereto as possible. Thus, the critical operation in forming the three cusps is the division of the tissue used to form the valve into three equal portions comprising the circumference of the valve stent. The prior art taught that the sewing of the tissue to the stent posts would be done only after the tissue had been mounted on a conical tool or similar device with indicia located at 120-degree intervals, and marking threads sewn onto the tissue at intervals corresponding to the 120-degree displacement of the stent posts. However, in forming the vertical seam below the marker between each stent post and the tissue the prior art taught that there was no precise practical method for locating the needle hole vertically below the marker corresponding to the 120-degree interval on the tissue. Any slight error in judgement in inserting the needle into the tissue opposite the stent would result in the tissue being slightly stretched after the stitch is made with each of the sewing holes on the stent posts. Any slight stretching of the tissue on the valve would result in a slight non-uniformity among the cusps in a tri-cuspid valve. In the actual manufacture of such valves by a skilled operator, as taught by the prior art the operator would place the pericardial tissue sewn together as a cylinder over the conical tool which had on it a means for marking the tissue into three equal portions. The operator would then place a bright color marking thread on the top of the tissue corresponding to the three equally spaced marks on the tool. The operator would then place the tissue on the valve stent and align the tissue such that the three marking threads were aligned with the three stent posts on the stent. The operator would then have to visualize a vertical plumb line extending down from the marking thread on the top edge of the tissue and running parallel with the center line running through the centers of each of the sewing poles in the stent posts. It is through the imaginary plumb line that the operator would insert the needle into the tissue for sewing the tissue onto the stent. Thus, it is apparent that a great deal of human judgement is involved in correctly aligning the tissue on the stent in order to obtain a uniform tri-cuspid, geometric shape.
In the prior art, there is one stitch in the pericardial tissue forming the cylinder at which the pieces join together at two edges. When the tissue is placed onto the stent, it is oriented in such a way that the stitch between the two edges of the tissue is placed flush with one of the stent posts in order that the material in the stitch does not participate in the opening and closing movements of the tri-cuspid valve. In the prior art, there is no such stitch at the remaining two stent posts when the tissue is placed over the stent, and it is found that sewing of the tissue in the vicinity of these remaining two stents is difficult due to the wrap-around stretching action which occurs between the tissue and the stent in the absence of a seam in the tissue at the stent. Furthermore, there is greater bulk in stitching material at one stent post only, the bulk of stitching material at the remaining two stent posts being somewhat less. Therefore, the stitching bulk has an unsymmetrical distribution in the valve of the prior art.
It is well known in the art that the thickness of the pericardial tissue used to make the tri-cuspid valve must be uniform and must conform to strict dimensional tolerances. Furthermore, it is well known in the art that a successful tri-cuspid valve cannot be fabricated from pericardial tissue with blood vessels in it. Therefore, in selecting and cutting tissue from bovine animal hearts, tissue having either nonconforming thickness or blood vessels had to be cut around or rejected. This has led to a very low yield in the prior art, corresponding to one usable pericardial tissue for one valve for approximately every 100 animals so used.